Fuel cells convert a fuel into usable electricity via chemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive electrolyte layer (such as Nafion™) such that together these three layers form what is commonly referred to as a membrane electrode assembly (MEA). A typical catalyst loading on the anode and cathode is about 0.05 to 0.4 mg of platinum (Pt) per square centimeter of support surface area (such as a porous carbon-based mat). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger power-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel cell stack. It will be appreciated by those skilled in the art that within the present context, any such arrangement of numerous individual cells arranged to increase the overall electrical voltage or current output are deemed to define a stack, even in situations where such precise stacked arrangement of the cells is not readily apparent.
Due to factors such as flooding or ice blockage in the diffusion media (DM) or flow channels, as well as H2 maldistribution within or across the MEA, some of the cells within a fuel cell stack may experience a reduced supply of H2 to the anode; this may occur during either startup or normal operation, and in extreme examples, the supply may be cut off altogether. A global H2 starvation in the anode (where the H2 supply is completely cut off) leads to a phenomenon known as cell reversal where the anode is polarized to a potential much higher than the cathode. When a small portion of the anode is cut off from the H2 supply, the portion of the cathode corresponding to the portion of the anode starved of H2 experiences voltage potentials that are higher than the oxidation threshold of certain key fuel cell components, such as the carbon that makes up the catalyst support layer. This in turn leads to carbon corrosion and a related performance loss or even electrical shorting in the effected cell.
Efforts to meliorate the effects of anode starvation and subsequent cell reversal have not been satisfactory. In one such effort, cell voltage monitoring (CVM) is used as a way to monitor the cell voltage change. Unfortunately, this monitoring only provides indicia of a hydrogen shortage event that has already developed within the stack. Moreover, placing CVM on every cell in the stack is costly because of the number of sensors, associated wires, and controller connections required to monitor and identify a non-systemic event in each cell. Another such effort may involve a catalyst that promotes preferential oxygen evolution reactions as a way to suppress competing carbon corrosion reactions; graphitized support strategies alone do not sufficiently reduce carbon corrosion rates under either global or localized H2 starvation issues that frequently accompany fuel cell system startup, shutdown, transient or flow blockage operational conditions.
Accordingly, a need exists for alternative methods and systems for monitoring and identifying non-systemic events within a fuel cell stack.